1. Technical Field
This invention relates generally to encoding pictorial imagery for halftone reproduction on binary display devices.
2. Background Art
As an approximation to continuous tone images, pictorial imagery is represented via a halftone image processing apparatus and process in which each input pixel is translated into a j.times.k pattern of recorded elements, where j and k are positive integers. A halftone image is reproduced by printing the respective elements or leaving them blank. That is, by suitably distributing the recorded elements.
Image processing apparatus and processes are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another measure of image processing apparatus and process merit is the tendency to produce visual artifacts in the output image that are not part of the original image, but are the result of the image processing, including moire patterns, false contours, and false textures. Moire patterns are false details created most often by the beating between two relatively high frequency processes resulting in a signal whose spacial frequency is low enough to be seen by the viewer. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible contour when the input image is truly a smooth, gradual variation from one to the other. False textures are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly and the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level.
FIG. 1 shows a schematic view of the electronic screening process. Signal X.sub.i represents the lightness or gray level information at a sampling point "i" of an image. Input signal X.sub.i of sample image pixels is compared with a series of threshold values C.sub.i selected in sequential order from a two-dimensional threshold value matrix, and a print/no-print decision is made. The series of threshold values and their arrangement within the threshold value matrix determine the gray scale range, frequency, angle, and other properties of the halftone pictorial image. By comparing the input signal X.sub.i with the threshold levels, j.times.k output signals O.sub.i are produced. A density pattern consisting of a combination of j.times.k elements is obtained by dividing each pixel into j.times.k elements and systematically printing them or leaving them blank. When the input signal X.sub.i exceeds the selected threshold value C.sub.i, the corresponding element is determined to have a print level (logic level "ONE"). FIG. 2 is a 4.times.4 threshold value matrix in which sixteen gray levels (plus all white) are obtained by sequentially increasing the number of elements which are printed, as shown in FIG. 3.
A problem exists with the number of density levels attainable with a limited resolution and acceptable screen frequency. One way to get more gray levels is to reduce the number of lines per inch and adoption of larger matrix dimensions, but this decreases the resolution and decreases the screen frequency to a visible level.
One known attempt to improve both gradation and resolution is the adoption of a small matrix for the resolution unit, and the adoption of a large matrix for the gradation unit, for example the so called "Improved Halftone" (IH) method. In the IH method, an 8.times.8 superthreshold value matrix is divided into four 4.times.4 submatrices. Having the same threshold value in the diagonal direction, sixteen density levels are output. FIGS. 4(a) and 4(b) show two 4.times.4 threshold value submatrices, and FIG. 5 shows an IH supermatrix formed of four submatrices, which are in turn each formed of four elements. FIG. 6 is a group of four adjacent supermatrices where submatrices according to FIG. 4(a) are labeled "a" and submatrices according to FIG. 4(b) are labeled "b".
When there are an odd number of elements turned ON, there is one more element in submatrices "a" than in submatrices "b". As shown in FIG. 7, this results in a 45.degree. apparent screen angle at a screen frequency having a 4(2).sup.1/2 element period. On the otherhand, an even number of elements turned ON, as shown in FIG. 8, results in a 90.degree. apparent screen angle at a screen frequency having a 4 element period.
While the IH method for determining a density level increases the number of available gray levels by printing the image at a higher screen frequency, there will be an increase in false texture artifacts (artificial changes in the image texture which occur when input gray levels vary slowly and smoothly and the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level) at any image portion where the number of black elements is small. This is due to the IH method producing a change in apparent screen angles between density steps.